The object of the present invention is a scintillation counter and, more particularly, a liquid scintillation counter that can efficiently detect both beta and gamma radiation.
Liquid scintillation counters are commonly used for counting samples that contain low energy beta particle emitting radioisotopes, especially tritium (H-3) and carbon-14 (C-14), which are widely employed radiolabels in biosciences. The ranges of low energy beta particles within the sample are generally at most some tens of micrometers. Consequently, the specimen to be analyzed is dissolved into appropriate an liquid scintillator so that the isotopes come into intimate contact with the molecules of the liquid scintillator and the emitted beta particles can interact with the scintillator substance. In these interaction processes part of the energy of each beta particle is transformed into a rapid light pulse comprising several photons and called a scintillation.
Typically, liquid scintillators comprise an aromatic solvent into which a small amount of fluorescing compounds called fluors are dissolved. The beta particle excites solvent molecules whereafter excited states migrate to fluors. Their subsequent de-excitation produces a scintillation.
The scintillations are detected with suitable photodetectors, preferably photomultiplier tubes (or shortly photomultipliers), which convert the scintillations into electrical pulses. Most often, two photomultipliers are employed that operate in coincidence whereby the scintillation pulse is accepted only if the two photomultipliers detect a pulse simultaneously. The purpose of coincidence operation is elimination of thermal background noise inherent in the photomultipliers.
Because of their low electronic densities liquid scintillators are relatively poor in detecting gamma radiation. Accordingly, there is a need to widen the applicability of liquid scintillation counters towards more efficient gamma detection. Two approaches for this are given in U.S. Pat. Nos. 3,898,463 and 3,944,832 that employ a gamma sensitive inorganic crystal scintillator having a hole or well with transparent walls to take the sample. The crystal is optically coupled to photomultiplier(s) and pulse shape analysis is used to distinguish between fast sample-originated pulses and slower crystal-originated pulses. An additional benefit is that the above detectors can detect, and thus eliminate, penetrating external background radiation to make low-activity beta counting possible.
Some limitations are associated with the above mentioned detectors, however. To reach the photomultipliers, scintillation photons from a sample have to traverse through the crystal that produces some attenuation and worsens the sensitivity. Furthermore, the required pulse shape analysis is rather complicated electronically and is limited to cases where the sample scintillations are essentially faster than crystal scintillations. Some modern liquid scintillators produce relatively slow pulses thus making pulse shape differentiation between sample and crystal less efficient. The present invention overcomes these drawbacks.